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VOL. XXXII psYCHOLOGICAL REVIEW PUBLICATIONS WHOLE NO. 152 <n OF PRINCE / 
NO. 5 1924 
MAY \ 7 1924 


Ly 
4S, 
£01 OGICAL seule 





Psychological Monographs 


BOTT EDeRy 


JAMES ROWLAND ANGELL, Yate University 
HOWARD C. WARREN, Princeton University (Review) 
JOHN B. WATSON, New York (J. of Exp. Psychol.) 
MADISON BENTLEY, University oF Itirnors (Jndex) 
S. W. FERNBERGER, University oF PENNSYLVANIA (Bulletin) 


The Intellectual Resemblance of 
| Twins 


BY 


v 
CURTIS MERRIMAN 


Assistant Professor of Education, University of Wisconsin 


PSYCHOLOGICAL REVIEW COMPANY 
PRINCETON, N.J. 


Acents: G. E. STECHERT & CO., Lonpon (2 Star Yard, Carey St., W.C.) 
Paris (16, rue de Condé) 





ACKNOWLEDGMENT 


The writer wishes to express his indebtedness to Dr. Lewis M. 
Terman and Dr. Truman L. Kelley of Stanford University; to Dr. 
V. E. Dickson of Oakland, California; to Dr. George E. Frasier 
of Greeley, Colorado; and to the public school teachers who gave 
their services so generously. 


Digitized by the Internet Archive 
in 2022 with funding from 
Princeton Theological Seminary Library © 





CONTENTS 


INTRODUCTION 


THE COLLECTION OF DATA 
1. Type of data 
2. Precautions used 
3. Sample forms for Beta tests 
4. Sample forms for teacher rating 
5. General summary of amount of data 


THE EFFECTS OF ENVIRONMENT 
1. Views of other investigators 
2. Treatment of present data 


Pee TECLECTUAL LEVEL OF TWIN 
~ POPULATION 

1. Twins with ages 5 to 14 inclusive 

2. Twins with ages 5 to 18 inclusive 

3. Sex variability of a twin population 


RELATION TO BIOLOGY 
1. Summary of previous studies 
2. The Biological evidence 
3. Statement of problem and proposed argument 
4. Treatment of present data 


GENERAL SUMMARY 
BIBLIOGRAPHY 
APPENDIX—ORIGINAL SCORE MATERIAL 


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INTRODUCTION 


At least three important biological problems are connected with 
the phenomenon of twinning: (1) The problem of twinning as a 
variation in the normal method of human reproduction. (2) The 
problem of sex determination. (3) The problem of the relative in- 
fluence of hereditary and environmental factors in human develop- 
ment. 

In this study no attempt will be made to do original work on the 
strictly physical aspect of the problem. We shall be satisfied with 
stating fairly the generally accepted biological views, and pointing 
out some of the implications of these views. The purpose of the 
study is primarily psychological and bears chiefly on the following 
questions : 

.I, What is the effect of environment upon the amount of in- 
tellectual resemblance of twins? 

2. Does the fact of twin origin and birth operate in any way 

to lower the intellectual level of a twin population? 

3. What light do the psychological data throw upon the current 

biological belief that there are two distinct types of twins, 
fraternal and duplicate? 











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Pere eCOLEECTION OF PALA 


Three possible sources of data presented themselves for con- 
sideration. First, it was necessary to secure data on the intellec- 
tual behavior of each twin when he worked alone. It was immedi- 
ately evident that the Stanford-Binet was the most desirable in- 
strument for this purpose. Second, it seemed desirable to have 
some kind of estimate given by some one who knew the members 
of the twin pair intimately. For this it was decided to obtain a 
teacher rating on a number of traits which are commonly accepted 
as primarily intellectual in nature. Third, it was desirable to sup- 
plement both of the above by group tests. For this purpose two 
types were used, The National Intelligence Test, and a modified 
form of the Army Beta. The latter was used to get as far away 
as possible from the verbal factors represented in the other tests. 

The following data were obtained: 


i, otantord-Binet tests for@s7 02... 105 pairs 
om, VeAbuen Estates 1OT)% tercakts deat. gO pairs 
mea finvebeta tests LOT, ys. Mees. ts 76 pairs 
4. National Intelligence tests for...... 143 pairs 


In the collection of the data, every known precaution was taken 
to insure the validity of the results. Of these precautions the fol- 
lowing may be mentioned: 

1. All tests were given by trained examiners. Besides having ~ 
studied the books and manuals on testing, each person had given 
a considerable number of tests under the personal supervision of 
a psychologist. It is believed, therefore, that the procedure was as 
nearly uniform as it was possible to make it. 

2. In almost all cases the Stanford-Binet test was given to the 
two members of a twin pair by the same person, the test of the 
second twin following immediately upon that of the first. 

3. All group tests were given to both members of a pair at the 
same sitting. 

4. Extreme care was taken to make sure that the children were 


4 CURTIS MERRIMAN 


actually twins. Strangely enough, two cases were found of chil- 
dren who were passing as twins, but were not twins. 

5. The twin population tested was limited to those found in the 
eight grades of the elementary schools. This was done because of 
the inadequacy of some of our tests when used above or below cer- 
tain age ranges. For example, the National Test was not designed 
to measure beyond a level represented by the brighter children in 
the eighth grade. The limitation of the survey to the eight grades 
also made it possible to avoid certain undesirable selective factors. 

6. Every possible effort was made to secure data upon every 
twin pair in a given school population. All the schools included in 
this study are co-educational. This made it as easy to locate twins 
of unlike sex as twins of like sex. Extreme care was taken on this 
point. The school principal or city superintendent cooperated by 
calling teachers’ meetings. Statements were made before the entire 
school. Diligent inquiry was made of the children themselves on 
the playgrounds. It was announced in the newspapers that a. search 
was being made for twins. It is believed, therefore, that factors 
which could have produced a systematic tendency to overlook 
cases which were not in the same grade, or which did not resemble, 
were pretty completely eliminated. In a later part of the study, data 
will be presented on the number of twins that appear in a general 
population, and on the relative number of like sex and unlike sex 
pairs. It is interesting to note that in the populations covered by 
this study the actual number of twins found agrees closely with the 
observed frequency in the general population. The same is true ~ 
as regards the relative number of like and unlike sex pairs. These 
facts give added weight to the statement that there was present no 
systematic tendency to overlook any cases. 

Before presenting the tables which give the results of the various 
tests, it is necessary to call attention to the test procedures em- 
ployed. 

The procedure used in the Stanford-Binet tests and the meshed 
of recording responses conformed strictly with the directions set 
forth in Terman’s “The Measurement of Intelligence.’’ All items 
of the tables are self explanatory except in the starred cases show- 


THE INTELLECTUAL RESEMBLANCE OF TWINS 5 


ing different chronological ages for the two members of a pair. 
This resulted from the tests being given at different times. 

The National Intelligence Test was given and scored according 
to the 1920 edition of the manual. ' 

The procedure used in giving the Army Beta test was somewhat 
modified. In its original form this test was given by pantomime 
because it was designed for use with non-English speaking men. 
Inasmuch as all the twins could speak and understand the English 
language, the following verbal form of instructions was used. 
Only the verbal instructions will be shown. The charts that were 
used were similar to the actual test figures, and the general pro- 
cedure was much the same as is used with the exercises of the Na- 
tional Test. The content of the tests and the method of scoring 
were exactly the same as for the original Army Beta. 


BETA TEST 


Arrention—Pencils up. I am going to show you some drawings like examples 
you will have to work in this examination. I will illustrate the examples, and 
you are to do all the examples in each test. Always, pencils up, when I say stop. 
bio La 

ATTENTION—Pencils up. Look at this drawing, marked Test I. Here are some 
alleys. Watch me as I try to run through them as quickly as I can without 
crossing any lines. I am going to go from this arrow to this one and must find 
- and mark the path. 

Now—Look at your papers. Test I. You are to trace the path through them 
as fast as you can, and be sure not to cross any lines, but find the way out. 

Go—Stop. (2 minutes) 

ge Th | 
ATTENTION—Look at this drawing. Here are some blocks piled up. Watch me 
while I count them. Then I put the 3 in the square here. Watch while I count 
these. Then I put the 9 in the square below. 

Now—Look at this drawing. Test I]. You are to count the number of 
blocks in each drawing as if you could see them all, and put the number in the 
square, 

Go—Stop. (2% minutes) 

TESF IIT 
ATTENTION—Here are some little circles and crosses. You see the first row with 
the circles. Some of the spaces are empty. I fill the spaces thus O, O, O, until 
all the rest of the spaces are filled. (E repeats with other rows.) 

Now—Look at your papers. Test III. You see the crosses and the little 
circles. You are to fill out the blank squares at the end, as the row has been 
started. Do all the rows. 

Go—Stop. (134 minutes) 


6 CURTIS MERRIMAN 


TEST IV 
ATTENTION—Look at these drawings. Here are some numbers with drawings 
below them. Below are rows of the same numbers with empty squares below 
them. Watch what I do. I put the right drawing with the number to which it 
belongs. Go all the way through each row. 

Now—Look at your papers. Test IV. You are to put the right drawing in 
every blank square below the number which it goes with. Go through every 
row to the end. 

Go—Stop. (2 minutes) 

TESTA. 
ATTENTION—Look at these figures. Some of the numbers on each side are the 
same. Some are almost the same but not quite. Watch what I do. 

Now—Look at your papers. Test V. If the two numbers are the same put 
a cross on the dotted line. If the two numbers are not the same do nothing. 

Go—Stop. (3 minutes) 

LES Lav 
ATTENTION—Look at this drawing. There is something missing. What is it? 
(Gets response and draws in figure.) (Repeat.) 

Now—Look at your papers. Test VI. In each picture draw in the part that is 
missing. Don’t try to make it pretty. Fix them all. 

Go—Stop. (3 minutes) 

TEST VII : 
ATTENTION—Look at this drawing. I put the figures into the square like this. 
They fit. Watch where the lines should be drawn to show how they fit. 

Now—Look at your papers. Test VII. You are to draw the lines which 
show how the little pieces fit into the squares. 

Go—Stop. (2% minutes) 

THe viel 
As this test was omitted in army use, it will not be used in this study. 


The teacher rating was secured by. means of a special blank of 
which the following is a copy: 


In each trait or characteristic named below compare this child with the 
average child of the same age. Then in the square before the name of the trait, 
place the figure I, 2, 3, 4, or 5. These figures are to be used with the following 
meanings: 

I—Very Superior to the average child of this age; 

2—Superior to the average child of this age; 

3—Average; 

4—Inferior to the average child of this age; 

5—Very Inferior to the average child of this age. 

( ) Memory. 

( ) Imagination (Ability to think about things not present to the senses.) 

( ) Reasoning (Ability to see meanings or to follow a complicated train of 
thought. ) 

( ) Judgment or common sense. 


C- 


THE INTELLECTUAL RESEMBLANCE OF TWINS 7 


( ) Resourcefulness in overcoming difficulties or attaining ends. 

( ) Originality (As shown by inventiveness or by ingenuity in finding ex- 
planations.) 

( ) Curiosity (As shown by inquisitiveness or eagerness to learn.) 

( ) Mechanical Ingenuity (Ability to think out mechanical contrivances. 

May exist without manual dexterity.) 

General Intelligence. 

Studiousness. 

Interest in objective things (plants, tools, etc.) 

Interest in books. 

Breadth and Variety of Interests. 


we Ne ee a” a 


The recorded grade for the teacher rating in Intellectual Traits 
is the mean of the various ratings. It should be borne in mind that 
the rating I is highest and 5 the lowest. 

The test results were then tabulated according to the following 
form: 


SEX GRADE CA. BINET BINET TEACHER BETA MT 
M.A. 1.Q. RATING 
Pains! Boy. tT 5-10 6-6 112 
F I 5-10 6-8 114 sets rhe 3 Ak 
Paite 2 Meee 2D 6-11 7-6 108 1.5 30 28 
F 2b 6-11 7-4 106 DD 45 44 


Table 1 shows the distribution of ages, the number who took 
the various types of tests and the total number of pairs studied. 
Appendix A gives the results of the various tests. The original 
scores are reported so that any one who cares to do so may make 
further study of the data. 


TABLE 1 
GENERAL SUMMARY OF DaTA 

AGE BINET TEACHER BETA N.LT. 

5 I 0) re) 0) 

6 9 8 3 I 

Gi II 8 7 2 

8 II II 7 14 

9 15 12 II 21 
10 9 9 8 16 
II 10 7 7 17 
12 15 12 12 26 
13 10 10 II 21 
14 9 9 8 13 
15 3 3 2 10 
16 2 I fe) I 
18 fe) 0 (a) I 
Totals 105 90 76 143 


THE EFFECTS OF ENVIRONMENT 


Galton made a general comparison of two groups of twins 
(Galton, Francis: Inquiries into Human Faculty, Everyman’s Li- 
brary, 1883, pages 155-172.) One group consisted of 35 pairs 
showing marked similarity in infancy, the similarity being some- 
times so pronounced as to cause confusion of identity. The second 
group was composed of 20 pairs of distinctly dissimilar twins. For 
the second group the environment had remained substantially the 
same. The excess of difference in the first case, and of resemblance 
in the second, was thought to give a measure of the influence of en- 
vironment. The persistence of similarities in the first case and of 
differences in the second was taken as a measure of the influence 
of nature. Galton quotes from many letters of parents showing 
how the original likeness or difference remained through child- 
hood to adult life. He summarizes as follows: 


“We may, therefore, broadly conclude that the only circum- 
stance, within the range of those by which persons of similar 
conditions of life are affected, that is capable of producing a 
marked effect on the character of adults, is illness or some acci- 
dent that causes physical infirmity. The impression that all this 
leaves on the mind is one of some wonder whether nurture can 
do anything at all, beyond giving instruction and professional 
training. There is no escape from the conclusion that nature 
prevails enormously over nurture when the differences of nur- 
ture do not exceed what is commonly to be found among per- 
sons of the same rank of society and in the same country.” (p. 
172) 

At least two comments are in place concerning Galton’s work as 
briefly outlined above: (1) He secured his results by the question- 
naire method—a method which investigators in the field of verbal 
report have shown to have many sources of error. (2) In spite of 
the imperfections of Galton’s method, his general conclusion as to 
persistence of nature has been fairly widely accepted. 


THE INTELLECTUAL RESEMBLANCE OF TWINS 9 


In 1905 Thorndike (Thorndike, E. L.: Measurement of Twins, 
Archives of Philosophy, Psychology, and Scientific Methods, 
Number One, September, 1905.) published a report of the meas- 
urement of resemblance of fifty pairs of twins in certain specific 
mental traits. The traits used and the resulting resemblances were 
as follows: 


1. Marking A’s on page of capital letters....... t= 09 
2. Marking words containing at or re ......... i 7k 
Beevarkine misspelled ‘words: oii. cise peiebie het s i. OO 
As povine addition: problems sox ek. ieciheidtons ele’ ti 275 
5. Solving multiplication problems ............ To=.54 
6. Writing the opposites of a set of words...... jer a8 8, 


Having found the resemblance for the twin population as a 
whole, Thorndike attacked the problem of the effects of environ- 
ment. His argument is as follows: 

“Tf now these resemblances are due to the fact that the two 
members of any twin pair are treated alike at home, have the same 
parental models, attend the same school and are subject in general 
to closely similar environmental conditions, then, 

1. Twins should up to the age of leaving home grow more and 
more alike, and in our measurements the twins 13 and 14 years 
old should be much more alike than those 9 and Io years old. 
Again, i 

2. If similarity in training is the cause of similarity in mental 
traits, ordinary fraternal pairs not over four or five years apart in 
age should show a resemblance somewhat nearly as great as twin 
pairs, for the home and school conditions of a pair of the former 
will not be much less similar than those of a pair of the latter. 
Again, 

3. If training is the cause, twins should show greater resem- 
blance in the case of traits much subject to training, such as ability 
in addition or in multiplication, than in traits less subject to train- 
ing, such as quickness in marking A’s on a sheet of printed capitals 
or in writing the opposites of words. 

On the other hand, 

1. The nearer the resemblance of young twins comes to equal- 
ing that of old, and 


10 CURTIS MERRIMAN 


2. The greater the superiority of twin resemblance to ordinary 
fraternal resemblance is, and 

3. The nearer twin resemblance in relatively untrained capaci- 
ties comes to equaling that in capacities at which the home and the 
school direct their attention, the more must the resemblances found 
be attributed to inborn traits.” 

For the detailed statistics, methods, and conclusions of Thorn- 
dike’s study the reader must go to the original monograph. The 
following quotation will suffice to show how Thorndike takes his 
stand alongside Galton on the general problem of nature and nur- 
ture. 

“The facts then are easily, simply, and completely explained by 
one simple hypothesis : namely, that the natures of the germ cells— 
the conditions of conception—cause whatever similarities and dif- 
ferences exist in the original natures of men, that these conditions 
influence body and mind equally, and that in life the differences 
produced by such differences as obtain between the environments 
of present day New York City public school children are slight.” 

However, neither the work of Galton nor that of Thorndike is 
entirely conclusive. In the first place, Galton depended upon a 
verbal report method. Thorndike knew the weakness of this 
method and made use of a series of mental tests, but tests of a 
kind far inferior to those at present available. Later experimental 
results have considerably discredited most of the tests he used, as 
far as the measurement of general intelligence is concerned. It is 
to be stated, however, that Thorndike made no claims for these | 
tests as measures of general intelligence. He plainly says he is re- 
porting only the results of the measurement of 50 pairs of twins 
in these specific abilities, viz., marking A’s, etc. He plainly states 
that he makes no claims as to what might be found for other 
mental functions, except as we may infer from the probably reign- 
ing likeness between abilities. In the second place, Thorndike 
doubts whether he has completely eliminated the possible effects 
of home and school influence. The Binet and other forms of intel- 
ligence tests appear to meet this requirement more satisfactorily, 
and are therefore used in this study. 

Because of these differences in approach, it could not be fore- 


oP ea“ 


THE INTELLECTUAL RESEMBLANCE OF TWINS II 


seen whether the results of the present study would or would not 
support the conclusion of Thorndike. Both because of its nature 
and its careful standardization the Binet test ought to throw new 
light on the intellectual resemblance of twins. For similar reasons 
the various group tests should also be of great service, especially in 
making it possible to deal with a large population in a relatively 
short time. 

Before presenting our results it is necessary to indicate the 
statistical procedures employed. All correlations were derived by 
the use of the Pearson product-moment formula. The tables will 
show the correlation coefficient, the probable error, and the number 
of cases (pairs) used in the calculations. The scores used in these 
calculations were as follows: 

1. For the Stanford-Binet correlations, the Stanford-Binet 
I.Q. as shown in Appendix A. 

2. For the Teacher correlations, the ratings as shown in Ap- 
pendix A. 

3. For the Beta correlations, the raw Beta scores as shown 
in Appendix A. 

4. For the N.I.T. correlations, the raw N.I.T. scores, as 
shown in Appendix A. 

Table 2 shows the coefficients of correlation based upon the 
Stanford-Binet I.Q. for the various groups of young pairs as com- 
pared with the old pairs. 


TABLE 2 
1. All pairs, 5 to 9 years, r = 809 + .032 with 47 pairs 
Mlnapaire TO tOnsO years, rs..757 21.037 i aso bea 
Difference = .042 + .048 
2. Like sex, 5 tOn GO (Years; f=, GOs. 7 anne ar edn 
bikes sex, 15°10 t0-16' years, ‘ric 865 cee 0272 “54.4877, 
Difference = .017 + .038 
3. Girl-Girl, Beto 0 year sy fa .018 1 Oc ne 1G, 0 4 
Gusi-Giri,, 10 to. 16:;years,'r — .o14/2.050,, 2 21, “ 
Difference = .101 + .056 
4. Boy-Boy, Sf 80 years, T0007 els amen a 10 
Bey iye. 1 10.tot years, © =5". 8008 8074 ay ar via 
Difference = .090 + .085 
Se Umike-sex,.+8 fo7'0 ‘years, £774 1064 a8 
Unlike Sex10 to.10 years, r =*.208, -=.137) " 20) * 
Difference = .476 + .151 


12 CURTIS MERRIMAN 


As far as the effects of age on twin resemblance are concerned, 
the present study confirms the conclusions of Thorndike. For all 
twin pairs with ages ranging from 5 to 9 years, inclusive, the cor- 
relation is +.809. For pairs with ages 10 to 16 years, inclusive, 
the correlation is +.757. The difference .042 is scarcely more than 
the P.E. of either measure. The P.E. of this difference, .048, also 
shows that no statistical significance can be attached to the change 
in correlation. In the case of the like-sex pairs the change from the 
young to the old pairs is —.o17; in the case of the boy-boy pairs 
+.090; and in the case of the girl-girl pairs —.101. In each case 
the P.E. of difference shows clearly that the slight change in cor- 
relation cannot be interpreted as indicating a difference in twin 
resemblance due to age. The results of the measures of the unlike 
sex pairs are not so clear. The drop from .774 to .298 is almost five 
times as great as is found in any other group. Three possible ex- 
planations present themselves. (1) Environment may actually op- 
erate to cause twins to grow more and more unlike. We have just 
seen, however, that environment has no such effect upon the other 
four groups studied. It is therefore reasonable to question the 
validity of this explanation and to look for some other explana- 
tion. (2) There may be inherited differences or likenesses that de- 
mand maturity to make them evident. This explanation, however, 
fails for the same reason that the first one did. Moreover, to accept 
this explanation would necessitate accepting the idea of a con- 
stantly changing I.Q. This is a debatable point, but the writer be- 
lieves that the preponderance of available evidence is in favor of 
the theory that the I.Q. remains relatively constant. Again, to ac- 
cept this explanation would necessitate showing why the change 
is a negative change rather than a positive one. (3) The change 
may be explained statistically in terms of the small population. 
That this explanation is probably the correct one, is suggested by 
two facts. The difference in correlation, .476, is scarcely more 
than three times the P.E. of the difference, .151. The best evidence, 
however, comes from the Beta and N.I.T. results which are to be 
shown later. For these tests the sharp contrast does not occur. It 
therefore seems reasonable to conclude that the statistical explana- 
tion is the most plausible one, and that as far as the Stanford- 


eae * 


THE INTELLECTUAL RESEMBLANCE OF TWINS 13 


Binet results are concerned there is no valid evidence that twin re- 
semblance becomes greater the longer the identical environment 
lasts. 

Table 3 shows the results of the Beta tests. 


TABLE 3 


1. All pairs, 5 to 9 years, r = .784 + .049 with 28 pairs 
Allepairs, 110 to 1Oyears, r= 064 co-osd tin a Ae 
Difference —= .120 + .072 
2. Like sex, CAtOmeOuVeal Sa ba) bac Oot mace TO 
Tike sex, — 10 to°16 years, f° .o42. .030. 20 
Difference = .079 + .043 
3. Girl-Girl, SrtOMmMOnyearse t <9 7001--ae Tl 2) a Or Be 
Girl-Girl, . +10 ,to/16. years). r == .806 == 032, “ . 16. “ 
Difference = .187 + .116 
4. Boy-Boy, Etto Or yeats, t=2.034°- .040/) yin Base 
Boy-Boy, FONCO BIOL YCarsatn— <7470 = g.0G0) me ante t3 | ve 
Difference = .087 + .093 
en Uphike Sexe Sto O-years, fr == 510 ch .1470 “ ‘12. “ 
Unlike Sex, 10 to 16 years, r= 643 = oor “ 19 “ 
Difference = .124 + .172 


When the methods already used in the examination of the Stan- 
ford-Binet data are applied to the Beta data, the same conclusions 
are justified. The differences between the younger and the older 
groups are either very small or can be explained statistically with- 
out necessitating the assumption that environmental factors have 
been operative. 


TABLE 4 
1. All pairs, 5 to 9 years, r= .797 + .034 with 54 pairs 
All pairs, #141 tOl IS years, £i5—0.075 22 O17 189 
Difference = .078 + .038 
2. Like Sex, Ertonroryearsars—-,040 +) OT2 bi 31. 
pce SOX at bi tO 16, veatsnf sa) O05 oo 4.022" 2 OT 
Difference = .o81 + .025 
3. Girl-Girl, KR toss years tesa O05) 2c..000.. i, 24 sre 
bitisGitl, © / 1. to 1S yearset—) 019 1.021. 937 4.“ 
Difference = .046 + .022 
4. Boy-Boy Sto. 10 years yf ",021) 04E) vy Yip Bee: 
Boy-Boy, ET ALOR TOV V CALS pita i055 O27 ee tee 
Difference = .026 + .049 
SPrmiike: sex 06 tO 10.years.Y = .76% = 000) 2s. 
(alike sex, 1 lvto 18. years; f= 834/25 Sodas > oR ae 
Difference = .0o81 + .079 


14 CURTIS MERRIMAN 


Table 4 shows the results of the National tests. The reader will _ 
note that the age ranges are different from those in Tables 2 and 
3. This change was made because the National Tests were not 
used below the third grade level. 

We find that the conclusions drawn from Tables 2 and 3 are 
fully supported by the data in Table 4; there is no evidence of any 
age difference in the degree of resemblance. 

Table 5 shows the results of the teacher ratings. 


TABLE 5 
1. All pairs, 5 to 9 years, r = 686 + .057 with 39 pairs 
All paits,.) (10 to.16 yearsora— 373 = Osi Tha 
Difference = .313 + .099 
2. Like sex, “to “Osyeats,.t <Te7oe) cO5ao) eas eee 
Like sex, 10 to.16 -years, -r ==1.565) ER08s os 30m | 
Difference = .220 + .008 
3. Girl-Girl, & tO SQcyears.it:, =. GES e020, © sve dee 
GitlGirl,*" 20°16 26 “yeats, toss -s2r 123") tones 
Difference = .392 + .126 
4. Boy-Boy, Sto~to Hyears, f 553d Ora Qe 
Boy-Boy, * 10 to “16 "years, 1/715 - 089 Sia = 
Difference = .181 + ».184 
By 2WnliketSex cs ‘to cor years; tia (0817-5000 mee J kOe 
Unlike: Sex, 10.40 16 -yéars, f/== 072) 2 14T, O ee 
Difference = .609 + .167 


It is not so easy to interpret the results shown in Table 5. The 
changes from .686 to .373 in the first group and from .788 to .568 
in the second and from .913 to .521 in the third are too much to 
be attributed to the size of population. Since the three changes are 
in the same direction, and that towards less resemblance, the writer 
believes the explanation lies in the better acquaintance that the 
teachers have with the older children. Since the children resemble 
more or less in physical appearance, including dress, teachers have’ 
been compelled to seek for all the differences in terms of which 
they might know one member from another. As the children ad- 
vance in the grades it becomes increasingly necessary for the 
teacher to be able to tell the children apart. It is therefore conceiv- 
able that this overemphasis on differences may operate to make 
the teacher rating a little less accurate for the older groups than 
for the younger. That this explanation may be correct is shown 


THE INTELLECTUAL RESEMBLANCE OF TWINS 15 


by a study of the correlation between Binet I.Q. and teacher 
ratings. 

For pairs 5 to 9 years old, r= .593 + .05, 

For pairs 10 to 16 years old, r == .536 + .05. 
When it is remembered that many studies have shown a correlation 
of approximately .60 between Stanford-Binet I.Q. and teacher rat- 
ings, it will be seen that the above explanation is at least plausible. 
As will be seen from a study of the differences and their P.E.’s 
the results for the other groups can be interpreted more easily in 
terms of small population. It is therefore to be concluded that 
while the results of Table 5 are not quite as convincing as were 
those of earlier tables, they may not indicate any serious disagree- 
ment with the results previously found. 


THE INTELLECTUAL LEVEL OF A TWIN 
POPULATION 


While the writer was collecting his data, he frequently met the 
questions: ‘Does not the general mental level of a twin group lie 
below that of the general population?” or, ‘‘Is there an intellectual 
handicap placed upon the individual who happens to be a twin?” 
The question is of sufficient interest to justify a brief examination 
of the present data. 

Table 6 shows the Stanford-Binet I.Q. distribution of the 200 
children composing the pairs with ages 5 to 14 inclusive, and 
Figure A shows the same data in comparison with Terman’s study 
of 905 unselected children. In Fig A the frequencies are treated on 
the percentage basis. 


TABLE 6 

1.Q. FREQUENCY 
BO=. 06 Piaamss wae seen 4 
O0=175 Hence ucternatawet ate treet ers 14 
WO wSS ace os,» Seer oeae 28 
Oe) OS rN cassuwindc es wie atin sete 49 
Oba TOS aicieioacin identities 54 
LOO-LV'5 cine eaeeieeal tae 33 
LEGH125 shale Seis eal eee IO 
120-13 5NE see eee PO eee 6 
$2661.45 ae Gl omaha cae eee 2 
Lotalevrise eaters cece 200 


It will be seen at once that there are some variations. The me- 
dian for the twin group is 97. Terman reports the median for the 
905 unselected children as 99. This difference of two points cannot 
be interpreted as showing twin inferiority, for other studies have 
shown some variation in median I.Q. For example, Pintner and 
Noble (Journal of Educational Psychology, November, 1920, p. 
716) in reporting a study of 450 pupils in grades one to five in- 
clusive, give a median I.Q. of 103 with an interquartile range of 
22. On the other hand, Chase and Carpenter (Journal of Educa- 
tional Psychology, April, 1919, pp. 179 ff.) in reporting the re- 
sponses of a composite group with ages 9 to 12 inclusive give a 


THE INTELLECTUAL RESEMBLANCE OF TWINS 17 


36-65 G6-75 76-85 86-95 S6-105 WG-5 WWE-'25 126-135 136-146 


2OO TWINS 2.0% 7.0% 14.0% 24.5% 27.0% 16.54 §.0% 3.07 1.0% 
TERMAN 3h 2.32% 6.6% 20.1% 33.9% 23.14 9-14 2.3% 0.87 
FIGURE A 





The heavy line diagram shows the distribution of I. Q. for the 200 twins and 
the dotted line diagram shows the same for Terman’s 905 unselected children. 


median I.Q. of 92 and an interquartile range of 15. It is to be noted 
further that the twin interquartile range is 18, and that 905 chil- 
dren of Terman’s study show an interquartile range of 17.7. If 
the twins of ages 6 to I1 inclusive are used, it will be found that 
they have a median I.Q. of 99.4. It is therefore concluded that 
these 200 twins show no greater departures from the normal than 
would many other groups of 200 non-twin individuals. 

The above comparison is based upon just the 1oo pairs falling 
within the age limits of 5 to 14 inclusive. This limitation was made 
in order to have the age range the same as the Terman study. This 
eliminated five pairs of ages above 14. Placing these back in our 
data and dividing our population into younger and older groups, 
by sex, we have the results shown in Table 7. 

Examination of the above data confirms the earlier conclusion 
that no handicap obtains. There are some differences that appear 


18 CURTIS MERRIMAN 


upon the surface: for example, the boy-boy pairs seem to excel the 
girl-girl pairs. This difference can be accounted for by the fact 
that the low girl-girl score is caused by the rather extreme retarda- 
tion of a very small number of the older girls. As the Stanford- 


TABLE 7 
TYPE OF GROUP NUMBER IN GROUP AVERAGE LQ. 
All Twin Pairs 105 
Pairs 5 to 9 years old 47 99 
Pairs 10 to 16 years old 58 04 
Like Sex Pairs 67 97 
Like Sex 5 to 9 years old 20 99 
Like Sex 10 to 16 years old 38 95 
Unlike Sex Pairs 38 05 
Unlike Sex 5 to 9 years old 18 08 
Unlike Sex 10 to 16 years old 20 92 
Girl-Girl Pairs 40 | 04 
Girl-Girl 5 to 9 years old 19 99 
Girl-Girl 10 to 16 years old 21 90 
Boy-Boy Pairs 27 100 . 
Boy-Boy 5 to 9 years old 10 99 
Boy-Boy 10 to 16 years old MTF IOI 
Boys of Unlike Sex Pairs 38 95 
Girls of Unlike Sex Pairs 38 95 
Total Boys—all pairs 92 96.9 
Total Girls—all pairs 118 04.4 


Binet is known to measure a little low for the upper ages, it is to 
be expected then that the younger groups would show a slight su- 
periority over the older in the test scores. The average of the boys 
in the unlike sex pairs is exactly the same as the average for the 
girls of the unlike sex pairs. The small difference between total 
boys and total girls can be accounted for by the relatively larger 
number of retarded girls in the older group. It is therefore to be 
concluded that the present data show no handicap placed upon the 
members of a twin population. 

The question of the relative variability of the sexes brings up 
still another aspect of our problem. Table 8 shows the Stanford- 
Binet I. Q. distribution for the boys of all pairs vs. the girls of all 
pairs. 

Examination of these results will show that the boys seem to be 
slightly ahead of the girls on most of the items, the widest dé- 


wT“ 


THE INTELLECTUAL RESEMBLANCE OF TWINS 19 





TABLE 8 
Binet 1.Q. Distribution of Boys vs. Girls 

1.Q. BOYS GIRLS 
50- 65 fe) 4 
66- 75 I II 
76- 85 14 17 
86- 95 24 25 
96-105 23 37 
106-115 15 17 
116-125 4 
126-135 2 3 
136-145 2 0 
Total 92 118 

Mean = 98.5 94.8 

Median = 07.3 06.5 

ri 87.6 84.5 

ChyS 108.0 104.5 

Oe OS 20.4 20.0 

o= 15.3 15.1 

Variability = 15.5 15.9 


parture being in the mean. On the other hand, the variability of 
the girls slightly exceeds the boys. That there may be a selection 
factor operating to produce a part of this difference is suggested 
by a little study of the scores made by the older children. In the 
15-16 year old group, there are 8 scores that lie below 96. Four of 
these were made by boys and four by girls. Three of the boy scores 
lie between 86 and 96 and one between 66 and 76. Three of the girl 
scores lie between 76 and 86 and one between 66 and 76. If these 
eight scores are dropped, the following results are obtained: 


88 BOYS 114 GIRLS 
Mean 08.1 95.3 
co 15.5 15.0 
V 15.8 15.7 


This brings the variability practically together and seems to sug- 
gest that there are no outstanding differences in favor of either 
sex. 

It is not the purpose of this study to determine the relative sex 
variability in a general population. At the present time this is a 
very debated point. A few studies have shown slight advantages 
for the boys. Others have indicated greater variability for the 
girls. Still others have shown little if any sex differences. The data 
here presented show such slight differences, that the writer is con- 


20 CURTIS MERRIMAN 


vinced that whatever is finally found to be the case with a general 
population, will likewise be the case with a large twin population. 

The answer to the question asked at the beginning of this part 
of the study, therefore, is, that there is no intellectual handicap 
placed upon an individual who happens to be a twin. 


RELATION TO BIOLOGY 


It is necessary to make clear the meaning that is to be given to 
certain terms that will frequently occur in this part of the study. 
The following are the usual definitions of the terms “siblings,” 
“fraternal twins,’ and “duplicate or identical twins”: 

1. Siblings. Children of the same parents, the mode of birth 
being the single birth. May be brother-brother, sister-sis- 
ter, or brother-sister relationship. 

2. Fraternal Twins. May or may not be of the same sex, are 
usually no more alike than are ordinary brothers and sis- 
ters, and are believed to be derived from two fertilized 
eggs. 

3. Duplicate or Identical Twins. Always of the same sex, are 
almost identical, and are believed to be derived from a 
single fertilized egg. 

It is evident that these definitions are built upon a certain as- 
sumption: viz. that there are two distinct kinds of twins, and that 
in some way these two kinds are determined by the mode of origin. 
It is also clear that the matter of sex plays a very fundamental 
part in this classification. That all authorities do not agree upon 
this may be brought out by a survey of some typical quotations. 


“The evidence in the case of the thirty-nine pairs of twins 
from whom we have extended physical measurements gives no 
reason for acceptance of the hypothesis of two such distinct 
groups of twins.” (Thorndike, Measurement of Twins, 1905, 


P. 44.) 


“Tn animals also there is much evidence, aside from that af- 
forded by the chromosomes, to be discussed below, in favor of 
the view that sex is internally controlled. . . . In the nine- 
banded Armadillo (Newman and Patterson, 1909, I9g10) one 
fertilized egg commonly gives rise to four new individuals, and 
the four are invariably all male or all female. Analogous in- 
stances of polyembryony are also known in insects. Human 
twins, if ‘identical’ (produced by the same egg), are invariably 


22 


CURTIS MERRIMAN 


of the same sex; if ‘fraternal’ (produced by different eggs) 
they may or may not be of the same sex. It would therefore 
seem that sex in such cases as these must be determined either 
in the egg before fertilization or at the moment fertilization oc- » 
curs.” (Sharp, Introduction to Cytology, 1921, p. 357.) 


“Resemblance will depend upon the identity of the hereditary 
primary constituents, on the similar combination of the germ 
plasm. Man himself affords a particularly good example in 
favour of this interpretation in the case of so-called ‘identical 
twins.’ It is well known that there are two kinds of twins, those 
that are not strikingly alike, and often very different, and those 
that are alike to the extent of being mistaken for one another. 
Among the latter the resemblance may go so far that the parents 
find it necessary to mark the children by some outward sign, so 
that they may not be continually confused. We have now every 
reason to believe that twins of the former kind are derived from 
two different ova, and that those of the latter kind arise from a 
single ovum, which, after fertilization, has divided into two 
ova.” (Weismann, The Evolution Theory, 1904. Vol. II, pp. 


44-45). ; 


“Tt is known that there are two sorts of twins. (1) The true 
or ‘identical’ twins are developed from a single original egg cell 
which at some very early stage divided to form two individual 
beings. These ‘identical’ or ‘duplicate’ twins have a nearly 
(though never an absolutely) identical germ plasm, are always 
of the same sex and resemble each other to an extraordinary de- 
gree. (2) The other kind ‘Fraternal’ twins are no more alike 
than brothers and sisters born at different times. They are de- 
veloped from two separate egg cells.” (The Journal of Hered- 
ity, October 1918, p. 262.) 


“Unlikeness in sex does not imply very much less difference 
in mental traits than that manifested by twins of the same sex.” 
(Thorndike, Measurement of Twins, 1905, p. 33.) 


“Biologists have for some time recognized at least two dis- 
tinct types of human twins: fraternal and duplicate.” (H. H. 
Newman, Biology of Twins, 1917,p.8.)  ~ 


“As is well known, twins are of two types: namely, those 
which are derived from two eggs ovulated simultaneously or 
nearly so, and those derived from a single egg which has formed 
two embryos.” (C. B. Davenport, Influence of the Male on the 


THE INTELLECTUAL RESEMBLANCE OF TWINS 23 


Production of Twins. In Medical Record for March 27, 1920.) 


There can be no doubt that a clear cut issue is raised. Thorn- 
dike can find no evidence to support the theory that there are the 
two distinct kinds of twins. Newman, Davenport and many others 
are just as certain that the classification is sound. It will therefore 
be the purpose of this part of the study to assemble all the evidence 
that can be obtained from the present data bearing on the question. 


THE BIOLOGICAL EVIDENCE 


It is probable that some of the earliest theories of twinning must 
have grown out of the observation of such twin births as are 
illustrated by the Siamese twins. It would be natural to assume that 
their origin would in some way be different from the normal single 
birth, Add to this the comparatively recent development of the 
study of cell growth and division, and we have the setting for 
some kind of cell-division theory to explain the occurrence of 
twin births. We should also include in the list of contributing 
factors three other forces that certainly have operated to some ex- 
tent to build up the present body of knowledge. There is the deep 
interest in the mechanism of heredity that has come as a result of 
the work of Weismann, Lamarck, Mendel, and others. There is 
the vast development of the use of statistics in all sorts of research. 
There is, in the third place, the interest in health and vital statis- 
tics. This last has impelled some of the leading nations to keep 
very elaborate birth and obstetrical records. It is from some of 
these last fields that much of the very convincing evidence has 
been obtained for the biology of twins. To illustrate this let us 
examine facts regarding the sex ratios of twins. 

On the supposition that twins originate always from two sepa- 
rately fertilized ova, and that it isa mere matter of chance whether 
an individual of a pair of twins is to be male or female, one can 
figure out the ratio that should obtain. This ratio would be 1: 2:1. 
That is, there should be one pair of boys, to two mixed pairs, to 
one pair of girls. Let us see how this works out in actual life. 

Since in each of these studies the sex ratio turns out to be ap- 
proximately 1: 1:1, there must be some other method of origin, 


24 CURTIS MERRIMAN 


SEX 0 PAS 


MALE MIXED FEMALE 
Frequency of occurrence 234,497 204,098 219,312 
Approximate ratio® 88 1.00 & 


(H. H. Newman, Biology of Twins, page 9. Quoted from Nichols.) 


pS BN 


MALE MIXED FEMALE 
Frequency of occurrence 1,118 1,193 1,023 
Approximate ratio® 93 1.00 8 


(Margaret V. Cobb: Evidence bearing on the origin of human twins from a 
single ovum, Science, Vol. 41, page 501.) 


since the above assumed method of origin would give a ratio of 
1:2:1. Biologists have argued from the Nichols data as given 
above to the conclusion that a part of the like-sex pairs must be of 
one-egg origin. This argument runs as follows: 


MALE MIXED FEMALE 
“1. Frequency of occurrence 234,497 264,098 219,312 
2. Approximate ratio I I I 
3. But if all twins are of two-egg origin, and 
sex is determined at time of fertilization, 
the ratio should be I 2 I 
4. That is, by law of chance, on basis of’ 
mixed pairs, we should have 132,049 264,008 132,049 
5. This gives excess of 102,448 Be Ape 87,263 


® Note: In the original the approximate ratio is given as 1:1:1. The writer 
has calculated to second decimal as above. 


If these two numbers are combined it gives a total like-sex pair 
excess of 189,711. When this is compared with the total number 
given by Nichols, 717,907, it is seen that the ratio is approximate- 
ly one-fourth. Nichols and Newman therefore conclude that about 
one-fourth of all twin births will be of the one-egg or duplicate 


type. 


substantially the same results. Miss Cobb does not use the terms 
fraternal and duplicate, but she does say that 28.4% of the pairs 
reported by her were presumably of the uni-ovular origin. 
French and German statistics on obstetrics (quoted by Dan- 
forth in Journal of Heredity, Vol. VII, p. 195 ff.) say 15% of 
twins are of uni-ovular origin. These figures are based upon the 
assumption that one can tell the origin by an examination of the 
membranes that enclose the child. It has been pretty conclusively 
shown that the single-membrane test is not infallible as a sign of 
single-ege origin. There have been cases of fused membranes 


The same method of treatment applied to the Cobb data gives 


THE INTELLECTUAL RESEMBLANCE OF TWINS 25 


which appear to be single, but are actually double. Many ob- 
stetricians also report the reverse condition: viz., multiple mem- 
brane, but one-egg origin. When allowance is made for these vary- 
ing factors, the French and German figures probably come very 
close to the figures previously quoted from Newman and Cobb. 

It does not fall within the province of this study to go extensive- 
ly into the literature of sex determination. For the data on this one 
must go to the work of Morgan, Doncaster, Bateson, et al. The 
evidence supports the theory that sex is determined at the time of 
fertilization and before the division of the cell. The bearing of the 
last statement upon the problem of twin origin is very important, 
since the whole explanation of the statistics noted above is based 
upon it. 

The arguments against the existence of two distinct types of 
twins center largely about the work of two men, Thorndike in 
America and Fisher in England. Thorndike approaches the matter 
wholly from the statistical angle, and submits a skewed unimodal 
curve as a summary of his argument. 


“The form of distribution of twin resemblance is apparently 
of the somewhat common type where a trait is very variable and 
has its mode close to an absolute limit of some sort. With all 


discretion in interpretation, however, one may be sure of (1) 
the general existence of close resemblance as the most frequent 
fact. (2) An extreme variability toward low resemblance, or 
even greater unlikeness than exists between two unrelated indi- 
viduals of the same sex and age, and (3) the absence of any 
sharp break into two species of resemblance.” (Thorndike, 
“Measurement of Twins,” p. 50). 

It is this last fact that stands out for Thorndike, and upon which 

he bases his conclusion that there are not two distinct kinds of 


twins. 


26 CURTIS MERRIMAN 


Fisher in England made a further study of the problem, using 
Thorndike’s data. He accepted the unimodal skewed curve as rep- 
resentative of the resemblance that exists between twin pairs. In 
discussing the cause for this type of distribution he says: “The 
fact that the observations examined critically show themselves to 
be a strictly homogeneous population, with correlation much larger 
than that between sibs, requires a new theory of the genetic con- 
nection between twins. It is here suggested that the facts may be 
explained by the supposition that twins ordinarily share the hered- 
itary nature of one gamete but not of the other.” (R. A. Fisher, 
“The Genesis of Twins,” Genetics, September, 1919, p. 496). On 
page 498 he further says: “If we suppose that in certain cases the 
ovum after maturation is induced to divide into two identical por- 
tions, which are fertilized by different spermatozoa, not only is the 
observed resemblance of twins numerically explained, but the in- 
fluence of the father is open to reasonable explanation.” 

The foregoing quotations show very clearly that there is no 
agreement in the outcome of the various lines of thinking. The 
biologists are pretty generally agreed that there are two distinct 
types of twins. Thorndike and Fisher, on the other hand, hold that 
there are not two distinct classes and that all the peculiarities of 
the resemblance curve can be explained in terms of a homogeneity 
of population, and that the phenomenon of sex determination is 
related in some way to the maturity of the ovum. The crucial ques- 
tion then becomes one of determining whether a twin population is 
heterogeneous or homogeneous. Do the psychological data of the 
present study throw any light on this question? 


THE PrRoposeD ARGUMENT 


Since the “two distinct species” theory is the more widely ac- 
cepted, let us assume that it is the correct theory and then list the 
principal claims that it makes, and the results that should follow. 
Having done this we can proceed to the examination of the data in 
hand to see how it confirms or rejects the claims made. 

1. There are two distinct types of twins, fraternal and duplicate. 

2. The fraternal, being of the two-egg origin, should show no 

greater resemblance than ordinary siblings, since each indi- 


THE INTELLECTUAL RESEMBLANCE OF TWINS 27 


vidual of the pair develops from a wholly independent ar- 
rangement of the factors for heredity in the germ cells. 

3. The duplicate, being of the one-egg origin, should show a 
very much higher degree of resemblance than the fraternal 
because each member of the pair develops from substantially 
the same arrangement of the factors for heredity in the germ 
cells. 

4. One of the real difficulties of the problem arises from the 
effort to tell whether a given pair belongs to the fraternal or 
duplicate type. Some have attempted to do this in terms of 
general resemblance of hair, eyes, facial features, etc. Others 
have sought to get away from the subjective factor involved 
in this method by using fingerprints, sole-prints, etc. These 
methods have not proven entirely satisfactory. There is, how- 
ever, one factor which stands unchallenged, and that is the 
sex-factor. Since sex is determined at the time of fertilization 
the duplicate twins must always be of the same sex. A crucial 
test of the theory will then be in the determination of the re- 
semblance of like-sex versus unlike-sex pairs. The resem- 
blance in the former case should run materially higher. 

5. This suggests the next difficulty. The group of like-sex pairs 
must by the law of chance contain a considerable number of 
fraternal twins. It was stated previously that about one- 
fourth of all twins are of the duplicate type. Since the like- 
sex pairs constitute roughly two-thirds of the number of 
pairs, then the number of duplicate pairs should approximate 
three-eights of the like-sex pairs. The differences, then, be- 
tween the resemblance curve of the unlike-sex pairs and the 
like-sex curve must be accounted for by the fact that approx- 
imately three-eighths of the group belong to the duplicate 
type of twin. 

The foregoing claims clearly show that we must make at least 

four attacks upon the data: 

1. What is the degree of resemblance shown by siblings? 

2. What is the degree of resemblance shown by unlike-sex 
pairs? How does this compare with the sibling results? 

3. What is the degree of resemblance shown by like-sex pairs? 


28 CURTIS MERRIMAN 


How does this compare with the sibling and unlike-sex pair 
results ? How does this bear upon the claim that there are two 
distinct types of twins? 

4. Do the present data lend themselves to such treatment that 
we can specify what particular group of like sex-pairs consti- 
tute the duplicate group? 


SIBLING DATA 


It was stated above that the coefficient of resemblance with sib- 
lings would furnish a valid check on our conclusions with refer- 
ence to fraternal twins. The writer has assembled sibling data from 
various sources and submits the following as typical: 

(1) From an unpublished study by Miss Grace Rensch, Stan- 

ford University. 


Palo Alto 167 cases lQO.r = + = .046 
San Jose Normal 61 cases LO, 9 =3 ae 
Palo Alto, 189 cases (paired) I.Q. r = + .61 + .035 
San Jose Normal 79 cases (paired) IQ. r= + .51 + .060 
Palo Alto, Sister-Sister 82 cases LOsr == 4) gi 2000 
Palo Alto, Brother-Brother 105 cases LQ. r = + .40 + .056 
Palo Alto, Brother-Sister 164 cases LQ. 5) See 
Average r = + .49 
(2) From Karl Pearson, Biometrika, Vol. III (1904). 
On THE Laws oF MENTAL INHERITANCE IN MAN 

BROTHERS t =) .64 r ==" 63 
Vivacity f= A7 t= ..40 ft ——G 8 
Self-Assertiveness 152 5G Y= P47 ti —744 
Introspection PS 1559 r = .56 n= 45 
Popularity te—".50 riot T= 452 
Conscientiousness t.=—=.50 SISTERS | BROTHERS AND SISTERS ~ 
Temper fis 51 Re we r—".49 
Ability Y saaG fA rv 52 
Handwriting Tss0,63 | On ee f=. Oe 
Average tT —62 fo 57 Yr 0 


General Average r = .52 


Many other studies have been made along similar lines. Some of 
these deal with mental traits and abilities. Others deal with strictly 
physical measurements. The fact to be remembered from all these 
studies is that sibling resemblance is universally stated to be very 
close to r == -+.50. None of the studies report significant varia- 
tions from this figure. 

The second question raised above has to do with the resemblance 


THE INTELLECTUAL RESEMBLANCE: OF TWINS 29 


of unlike-sex pairs, and its relation to the resemblance shown by 
siblings. The following shows the results of this part of the study: 


RESEMBLANCE OF UNLIKE-SEX PAIRS 


1. Stanford-Binet r= 504 = o8f - - 38 cases 
2. Army Beta t-=3).732 =) 050) —= = salecases 
Zhe CANS ol be Ae r = 0607 = .025 - - 51-cases 
4. Teacher estimate T=, 200) ce LO2- = = 98 3 7acases 


These figures furnish convincing evidence for the relation be- 
tween fraternal twins and siblings. It was said in an earlier con- 
nection that fraternal twins are siblings as far as heredity is con- 
cerned. We have just seen that the sibling resemblance runs close 
to the r == +.50 mark, and the present figures for unlike-sex pair 
twins, especially the Stanford-Binet scores, give practically the 
same amount, (r = .504). The Beta Figure (r = .732) and the 
N.I.T. (r = .867) are above the theoretical .50, and the Teacher 
estimate (r = .266) is below the expectation. At first thought 
these departures would seem to argue against our thesis. When 
these figures are compared with the figures for the like-sex pairs 
it will be seen that there is a plausible explanation for the apparent 
disagreement. Let us therefore turn to that comparison. 

The following figures show very plainly that there is a difference 
between the like-sex group and the unlike-sex group. 


LIKE-SEX VERSUS UNLIKE-SEX PAIRS 
Stanford-Binet 


Like-sex f ==) C07n==50201 =) 58 O7eCases 
Unlike-sex Cit S04e > Ors (= <= 20; CASES 
Difference = .363 + .083 
Beta 
Like-sex f==.008 = 20177.= ¥-" 45. cases 
Unlike-sex Gace 732s O50) =) =. ST pcases 
Difference = .176 + .058 
IN ad bad B 
Like-sex ft = .925 = .000 = > = 02 cases 
Unlike-sex r = 67 + .025 - - S51 cases 
Difference = .058 + .026 
Teacher 
Like-sex Trs=".054 57.053) =. = 63 Cases 
Unlike-Sex Te 1.200 == 1025) - re Cases 
Difference = .388 + .114 


Since it was pointed out in the proposed argument that this 
would be a vital point, let us examine these results more in detail. 


30 CURTIS MERRIMAN 


In the Binet results the like-sex resemblance is .867 while the un- 
like-sex resemblance is .504. This is a difference of .363 + .083 in 
favor of the like-sex groups. The Beta resemblance shows a dif- 
ference of .176 + .058 in favor of the like-sex groups. The N.I.T. 
difference of .o58 + .026 is not as large as the others, but since the 
difference is in favor of the like-sex pairs, and since a small dif- 
ference between high r values may be quite significant, it can hard- 
ly be argued that the N.I.T. evidence contradicts the Binet and 
Beta evidence. The teacher rating results show a difference of | 
.388 + .114 in favor of the like-sex groups. All of the evidence 
thus far, therefore, supports the biological claim that there are two 
distinct kinds of twins. 

This conclusion was reached by a study of resemblance in terms 
of correlation coefficients. There is still another way in which the 
same data can be treated, viz., in terms of differences in gross 
scores. Largely because of the size of population available, this part 
of the study will be based upon the Stanford-Binet I.Q. and the 
National Scores. 

Table 9 gives the distribution of arithmetical difference in Binet 
I.Q. scores for the four groups studied. 


The data of Table 9 can be studied in two different ways. First, _ 


we can calculate the mean 1.Q. difference for certain types of 
population. The difference between these means will be a measure 
of the homogeneity of population. The results of this plan of at- 
tack are as follows: 


Mean 1.Q. difference, unlike-sex pairs = 9.52 + .85 
Mean I.Q. difference, like-sex pairs == 6.05 + .45 
Difference == 3.47 + .96 


Since the difference is 3.6 times its P.E. the evidence from this 
direction favors the theory that we have to deal with two types of 
population. 

The data of Table 9 may also be treated graphically as shown in 
Figure B. It is seen at a glance that there is a decided difference 
in the types of curves. The siblings and unlike-sex curves resemble 
in a general way. Each starts with a low frequency for zero differ- 
ences in I.Q. This agreement seems to lend some graphic proof 
for the biological claim that unlike-sex twins have the two-ege 





ee tested i 18h] 
JRE 


ms LAN Ae tee ty 
(ae 


THE INTELLECTUAL RESEMBLANCE OF TWINS 31 


Bag Pt att prrereners wnat FETT TT ET TE 
esa siuincs. | {| | | | | tt ttt i tt 

waiay + |e a ietaneriod Pps eT Lal tebe Ie teh Pn lee Te | 

RT Mie Chaka eas 

WAT | I 





B 
ie 
ee 
Ne 
ice 
Re 
pale 
mila 
ee 
ol 


Pee N INS eae 


[| 
‘ FECES NSE 


Pepe siete Ss G6 ¢ 6 9.10 WM 2 1s 4 AS 16 17 18 19 26. 21 22 pa polar scariae es nc ai 32 33 34 355 36 37 - 


FIGURE B. 


origin in the same manner that siblings have. On the other hand 
the like-sex curve starts with its highest frequency for zero dif- 
ferences. This would seem to conform to the biological claim for 
“duplicate” twins. | 

Similar facts are revealed by the N.I.T. data, as shown in Figure 
C. There is one difference to be noted in the construction of the 
curves of Figures B and C. In Fig. B the separate scores are plot- 
ted. In Fig. C, the scores are grouped as indicated at the bot- 
tom of the figure. This was done for two reasons. In the first place 
the score range for the N.I.T. data is much greater than for the 
Stanford-Binet. The relative frequencies are therefore smaller and 
it becomes difficult to see the form of the curve. In the second 
place, it is necessary in a later part of the study to use the grouped 


45. = 5d 





32 CURTIS MERRIMAN 


TABLE 9 
DIFF. ALL PAIRS LIKE-SEX UNLIKE-SEX SIBLINGS 
fe) 9 8 I 6 
I 6 6 (o) 12 
2 10 6 4 8 
3 9 5 4 II 
4 10 7 3 13 
5 9 7 2 9 
6 9 6 3 12 
7 2 2 I 10 
8 6 3 3 13 
9 3 3 fe) 6 
Io 4 2 2 13 
II 4 2 2 6 
12 2 fo) -: 8 
13 5 3 2 6 - 
14 4 I 3 3 
15 2 0 2 9 
16 2 2 a) 6 
17 oO ‘0 fe) 14 
18 I I re) 4 
19 I I Oo 5 
20 (6) (a) 0. 6 
21 2 I I 9 
22 fa) re) oO 4 
23 I I fe) 6 
24 oO oO 0) fo) 
25 I oO I oO 
26 Oo Oo Cy) ag 
27 ce) fo) fe) 4 
28 oO fe) te) mie 
29 O fe) ) I 
30 0 oO fo) 3 
31 I oO I) 
32 0 Oo 0) 
33 fo) oO 0) 13 
34 I .e) 1) 
40 oO re) 0) 
41 to 50 o oO oO 2 
105 67 38 234 


form of the data, and for convenience the same grouping is used 
at this time. . 

If this distinction in type of curve is a real one it should be 
shown by the tests that have been devised for homogeneity and 
curve fitting. The best test for this purpose that the writer has been 
able to find is found in the work of Karl Pearson and W. Palin 
Elderton who have published a formula and complete tables for 
testing curves where two separate populations are concerned. The 
formula and its method of use can be seen from the following 


oan 


THE INTELLECTUAL RESEMBLANCE OF TWINS 33 


DISTRIBUTION OF DIFFERENCE 
IN WN. J.T. 


92 LIKE- SEX PAIRS 
——-— 5! UNLIKE-SEX PAIRS 


o-s 6-20 21-35 36-60 €1-€0 81-100 (01-120 121-170 
LIKE-SEX 7 16.3 42.4 26.1 10.8 
INLIKE - + ILT 23.5 23.5 176 
FIGURE C 





quotation: “If N and N’ be the sizes of two samples and the cor- 
responding frequencies: 


Pits alas eu eatene Tien aa wire ie 
ete APE AY S| Miele ha aS ee ey fc 
where f,, f’, are the frequencies falling in the p™ category, then if 
wes 
ae 
Bee cope is Nanci 09 
( ieee is) 


be calculated, the probability, P, that the observed or a greater di- 
vergence between the two series would arise from sampling the 
same population is obtained by determining P from X by my 
method of testing “goodness of fit.’ This method was first pub- 
lished in Phil. Mag. Vol. 50, p. 157, 1900. The shortest method of 
actually determining P is by aid of Palin Elderton’s tables for P 


34 CURTIS MERRIMAN 


with argument X? issued in Biometrika, Vol. I, p. 155, 1902.” 
(Karl Pearson, Biometrika, Vol. 10, p. 92.) 

The following pages (35-36) show the application of the Pear- 
son formula and Elderton tables to the Stanford-Binet and Na- 
tional Intelligence Test data. 

Since the S-B data show a probability of 27 to 1 that the popu- 
lations are different, and since the N.I.T. data show a probability 
of 40 to 1 that they are different, it is clear that the biological 
claim for two distinct types of twins is Ley supported by the | 
data on intellectual resemblances. 

It was stated at the beginning of this study that the neoblen 
would be limited to intellectual resemblances. It is pertinent to ask, 
however, at this point what would be the result if physical resem- 
blances were studied by the same methods that we have been using. 
Thorndike gives on page 43 the correlations for a number of such 
measurements but he gives only the correlations for the twin 
group as a whole. He makes no distinction between like-sex and 
unlike-sex groups. His gross measures are given, however, in 
table 8 on page 37. The writer took these measures and calculated 
the resemblance in height and cephalic index. The results are as fol- 
lows: 


HEIGHT CEPHALIC INDEX 
a. 39 twin pairs c= o778 ee 
b. 9 unlike-sex pairs r = 609 + .176 if) ——yedOle toma 
c. 30. like-sex: pairs... r — 821 2 042 r = 800 + .045 
Note: a = Thorndike’s computations; b and c = the writer’s. In b and c, 
0745 (ite) 
P, E. = ————————— 0n account of small population. 
Vn — 3 


It is hard to explain the difference in resemblance by any other 
theory than that of a real difference on the basis of likeness of sex. 
It must be admitted of course that the small population of the un- 
like-sex group and the resulting large P.E. reduces the value of 
the results somewhat but they certainly are in substantial accord 
with the results of the study of intellectual resemblances. 

Up to this point, the study has proceeded in terms of the entire 
group of like-sex versus unlike-sex pairs. The conclusions are quite 
definitely in favor of the two distinct classes. We can not be en- 


35 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


‘JUdIOJIp 

aie suoljejndod om} yey} I 0} saoueyo Zz A[YySno1 10 
cbt Lto' = g saqey, uoyopyy Aq aousy AA 

+fQ 11 = 

GO VOU ea Chee eC eee 

tsgvoo: X ,NN = 


2X 





pes > ate 
( Ba oe! ) 
T eae 
CN N) pts sXe 
( ) 
(545)/eN/J—N/J) 690700" $g0000° go00000" 10L100° $61000° $6S000° (8) == (£) + (L) 
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IN/ ggLor £gzo" gzSor b6gz" ggtz: gs 18 (5) = gf + (z) 
N/j 0000" grro: L6So° bbor’ be rf- glLr- “#(¥) = L9~ (1) 
$o1 St 5 14 9 gi of bb (£) = (%) + (1) 
gt * I zt II any: wl (z) XaS-o41[U) 
Lo j fo) £ + L It am (1) X9S-O4I'] 








NOILVINdOd HNVS AHL WOU SHTANVS 
auxV SNOILNGINLSIG ADNANAAAIG LANIG-GYOANVLS OML YAHLAHM ANINUALAC OL SNOLVINOIVO 


CURTIS MERRIMAN 


36 


‘JUIIIIP 
aie suorepndod ay} yey} I 0} saoueyd oF AjYysno1 JO 
bzo' = g saqe yy, uoylopyy Aq souay AA 
+ II‘'QI = 
EOVeVEOO els ec 
C6EEr"Loo’ XK WNN 


I a | 














ae ee ) 
( Ase 3 ) 
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a Jt I £ % 8 61 gf 1$ I (€) | =(z)+(1) 
1$ ey I t £ 9 6 rat ZI 9 (t) | xas-aytTUG 
76 J fe) I I Tt OI +z 6¢ $1 (1) XOS-O¥I'T 
abe Mies. [a olt-1e1--| ocr-iors | Poorzrg— | 08-19 es eed ES BS a eed | 








NOILV1NdOd ANVS AHL WOU SHTAWNVS 
AuYV SNOILNGINLSIG ADNANAAHIG “L T'N OML YHHLAHM ANINAALAG OL SNOILVTNOTVO 


THE INTELLECTUAL RESEMBLANCE OF TWINS 37 


tirely certain of this, however, until we have made a more intensive 
study of the resemblance of the various pairs. Thorndike’s method 
of doing this is fully discussed on pages 45-46 of his monograph. 

“Our problem is to measure accurately the resemblance found 
in each pair of twins, and so to ascertain the form of distribution 
of the group with respect to resemblances. Suppose, for instance, 
that of forty twins, we found twenty to resemble each other prac- 
tically perfectly, the coefficients being: 92, 93, 94, 94, 94, 95, 95; 
95, 95, 95, 96, 96, 96, 97, 97, 97, 97, 98, 99, and 99, and the other 
twenty to resemble each other as follows: 17, 18, 24, 29, 32, 37, 
37, 38, 40, 41, 41, 41, 42, 42, 44, 45, 46, 46, 51, and 62. It would 
be clear that there were two distinct types of twins. 

To measure accurately the resemblance of an individual pair is, 
however, very difficult. The most serviceable measure which I am 
able to devise for a single trait is the Pearson coefficient, using 
each individual twice in the calculation. That is, if the deviation 
measures are: 


First) member:of pair aw fw wiles: 6, 
eecond membenot paiticsie:a <a: ie 
Then the =xy = 18 + 18 
2x? = 36 + 9 
AY An Ont: 36 
. ands qian" 304 So 00 
The objections to this measure are that when both members of 
the pair are near the central type, it may misrepresent the real re- 
lationship and will be much distorted by accidental errors in the 
deviation measures of an individual. Thus, suppose that in a case 
where the variability of the trait is 10, two twins score —1 and 
—2. Their r as calculated will be —.80, but they are really very 
much more alike than this figure would lead us to think. Suppose 
by accidental error the first member scored —2 instead of —1; 
then their ris —1.00. Suppose him to score —1; the resemblance 
is —.80.” (Thorndike, Measurement of Twins, pp. 45-46. ) 
Thorndike regarded these objections as of sufficient validity to 
warrant other interpretative calculations, briefly indicated in the 
following quotation: ‘“‘For each pair in each trait is given the r, 


38 CURTIS MERRIMAN 


the amount of deviation from the central tendency of that one of 
the twins who deviated from it most (this is given in each case as 
a multiple of the median deviation for the trait in question), and 
the difference between the two twins’ measures (this is given in 
each case as a multiple of the median difference of all the twins 
in the trait in question).”’ (Measurement of Twins, pp. 48-49.) 

These measures were all assembled in the form of a distribution 
table and Thorndike concluded, ‘‘There is no sign in any of these 
of a sharp separation into a group of ‘duplicate’ twins with r’s 
approaching 1.00 and differences approaching 0, and a group of 
r’s approaching 4o and differences centering around a point well 
above the median difference for twins.” This conclusion is un- 
doubtedly justified if the figures and distribution of his tables 
13 and 14 are accepted without question. The writer cannot, how- 
ever accept them without question. There is first the value of r 
when the twin measures are near the central tendency. There is in 
the second place the introduction of a subjective factor. On page 
48 Thorndike speaks of interpreting or considering one measure 
in the light of certain other conditions, and in a footnote on page 
49 he speaks of his method of correcting r’s by adding certain 
amounts to the obtained r if its value lies between —1.00 and 
-+-.60. He also speaks of altering the correction somewhat in view 
of the amount of the deviation. How much this subjective factor is 
worth, the writer does not attempt to say, but its very presence 
raises the question of the validity of the results. 

In the present study a different method of calculating the in- ~ 
dividual pair resemblance has been used. This method is believed 
to avoid the difficulties just pointed out. The formula used is a 
special adaptation of the Pearson product moment formula and 
is as follows: 

(Difference in scores)” 


SS 
2 times o” of entire population 


The derivation of this formula is briefly as follows: 
x yi 


THE INTELLECTUAL RESEMBLANCE OF TWINS 








x? Z2xy zy? 
Expanding o,° = ——— — ——— — 
p § % N N N 
=x? xy? 
But o,? = ——— and o,? = ——— 
N 
Z2xy 
Lee xy 
Substituting 0.7 = 1 — —— 4 I 
N 
Z2xy 
Oa1G; =2x 
But ee Z 727, 
N No, gy 
oo = 2—a2r 
4 
She ee es 
2 


In special case let D = difference in gross scores. 


D 
Then. das 
Co 
D 2 
>! ) 
Ratitige botkriics bye sa te) 
ulti oth sides —_ ,_ —_—-_ —->='—s ———— 
ply y N N N 
=D? 
Whence o,7~— 1/¢e* 
N 
= 1/o” oD 
op" 
Substituting r = Ir — —— 
Plies 


: ( Difference in scores)? 
orinspecialcaser == Yo — —W———— 


2 times o? 


39 


40 CURTIS MERRIMAN 


The following computation will show the contrast between the re- 
sults according to the method used by Thorndike and the one here 
proposed. Suppose we have two twin pairs whose Stanford-Binet 
scores are for the first pair 97 and 100, and for the second pair 
99 and rot. Let us suppose also that the mean score for a twin 
population is 98 and the standard deviation is 15. The compara- 
tive results will then be: 

(1) By Thorndike method, r first pair = —.80 and r 2nd pair = +.60 

(2) By proposed method, r first pair = +.98 and r 2nd pair = +.99 
It is quite evident that a very substantial difference will be made 
in the distribution tables when such results are assembled. In the 
above instances, the —.80 and the +-.60 will go into nearly oppo- 
site parts of the table or curve, while the +.98 and +-.99 will lie 
near together. When it is remembered that the original scores were 
_ very nearly alike, the second alternative seems the more reasonable. 
It was noted above that Thorndike introduced a subjective factor. 
Since the present study is based upon a rigid use of the method as 
just stated, it is believed that the subjective factor is pretty com- 
pletely eliminated. 

Tables 12 and 13 show the results of the application of this 
formula to the Stanford-Binet data, and for comparative purposes 
to one item of the Thorndike physical measurements. The first. 
three columns give the distribution of r’s for all twin pairs, like- 
sex pairs, and unlike-sex pairs respectively, the calculations being 
based upon I.Q. differences. The last three columns give the cor- 
responding data for calculations made from the original cephalic 
index measures as given by Thorndike on pages 37-39. | 

The outstanding fact shown by Table 12 is the more pronounced 
resemblance shown by the like-sex group. Of 67 pairs in this group 
45 show a resemblance falling within the .go0 to I.00 range. In the 
unlike-sex group 17 out of 38 lie within the same range. It should 
be noted also that the cases of extreme negative correlation belong 
to the unlike-sex group. Table 13 shows the force of this argument 
more clearly. This table shows the distribution of the cases fall- 
ing within the .go to 1.00 range. If we accept the range of .99 
and 1.00 as amounting to practical identity we find that 20 out of 
45 pairs of like-sex pairs are practically identical, while but 5 out 


THE INTELLECTUAL RESEMBLANCE OF TWINS 41 


TABLE 12 


Distribution of Resemblance in Individual Pairs as Measured by 


Teed a 








ALL LIKE UNLIKE 
ALL LIKE | UNLIKE ey rae oe 
R bel re habe CEPHALIC | CEPHALIC | CEPHALIC 
se po ee INDEX INDEX INDEX 
Less than —.g1 2 2 
—.go to —.8I ° 
—.80 to —.7I ° 
—.70 to —.61 ° 
—.60 to —.s1 ° 
—.50 to —.41 ° I I 
—.40 to —.3I I I I I 
—30 to —.2I1 ° 
—.20 to —.II I I I I 
—.1o0 to —.olI ° 
—.0o to -.09 2 I I 
+10 to -+.19 fe) I I 
+.20 to -+.29 2 a) 2 I I 
+.30 to +.39 fe) I I 
+.40 to +.49 7) 2 I I 
+.50 to +.59 6 I 5 8 2 I 
+.60 to -+.69 4 3 4 
+.70 to -+.79 8 4 4 2 2 
+.80 to -+.89 3) 8 4 4 4 
+.90 to -+1.00 62 45 17 22 18 
Total Lbror Abi 675) (ae stay] Sa eg eee 8 





ALL LIKE UNLIKE 


ALL LIKE | UNLIKE es pias ae 

R take Soe i ae CEPHALIC | CEPHALIC | CEPHALIC 
eine ped pe INDEX INDEX INDEX 

go 

gi 

92 9 6 3 2 2 

93 

94 I I 

95 9 7 2 I I 

.96 4 4 

97 fe) 7 @ I I 

98 9 5 4 5 5 

-99 10 6 4 4 3 I 

1.00 15 14 I 4 3 I 

Ba] 





42 CURTIS MERRIMAN 


of 17 unlike-sex pairs can be so regarded. The evidence that has 
just been cited from the Stanford-Binet part of the tables can be, 
in the main duplicated with reference to the cephalic index part 
of the tables. 

One other consideration remains. Do the present data lend them- 
selves to such treatment that we can specify what particular like- 
sex pairs make up the group of duplicate twins? In our study of 
the Stanford-Binet I.Q. differences it was stated that the peculiar 
shape of the curve was produced by the presence of approximately 
2s duplicate pairs. When this study was first planned it was hoped 
that the internal evidence of the data might enable us to identify 
these pairs. The writer has, however, been unable to devise any 
method of studying the curves or the scores that will certainly 
point out the duplicate pairs. It is exceedingly interesting, however, 
to follow up certain evidence that is contained in the verbal reports 


TABLE 14 


Scores AND SCORE DIFFERENCES OF THE PAIRS THAT WERE REPORTED AS SIMILAR 


























i Differ- Ratin Beta AR i 
Pair Ta ence Teacher differ: "| Beta Differ- N.LT iffer- 
Eee 1.Q. rating aye: scores hee Scores SE 
5 | 104— 96 8 2.3—2.2 De Yb) ihe sce a ta 
8 | 110—106 4 7b 5 O08 5.0108) pees sat Tah Ns boesbiest 
Io | I107—II0 3 2.5—2.4 ol 25—18 9 ROAN Ne Bice 
12 | 66— 62 4 3-7—3.6 as Ph ere ceedtl > | ON Etats 
13 | 93— 93 ° 3-I—3.1 0.0 7—16 9 + ao lerinte 
22 | 102—I00 2 2.6—3.0 av ya Wy oatte ie a Bes e 
28 | 102—102 ° 2.9—2.7 py) 48—46 2 140—196 56 
29 | 104—I109 5 3-I—3.1 0.0 45—58 13 I14—122 8 
30 | 103— 96 % 3-:0—3.0 | 0.0 37—44 7 48— 46 2 
36 | 123—118 5 3:0—3.0 | 0.0 61—So0 II | 268—286 | 22 
46 | 135—127 8 2.3—2.3 | 0.0 46—60 14 | 246—238 8 
58 | 112—106 6 2.7—3.0 a 74—72 2 | 250—232 18 
76 | 67— 73 6 3-4—3-4 | 0.0 54—51 3 | 126—142 | 16 
77 | 85— 85 ° 2.4—2.4 | 0.0 51—63 129 5 ees ; 
84 | 75— 72 3 2.7—3.1 4 4I—20 OE 94—124 | 30 
86 | 88—107 19 3-I—3.1 0.0 77—80 3. | 264—254 10 
88 | 10g—122 13 2.2—2.2 | 0.0 84—78 i te 
94 | 104—103 I 3:0—3.0 | 0.0 80—78 2 | 296—296 ° 
95 | 98— 98 ° 2.6—2.9 By 74—69 § | 298—320 | 22 
100 | 80— 85 5 3-5—3+3 2 67—81 7 aed Re fe 
104 | 83— 83 fe) 3.2—3.3 i 69—73 4 | I98—208 Io 
107 | IOI—I07 6 1.7—1.6 I i Adal 68 gos 
| 





THE INTELLECTUAL RESEMBLANCE OF TWINS 43 


of the examiners. Each examiner was asked to report whether 
the members of the twin pair being studied resembled each other 
closely enough to frequently cause confusion of identity. Unfor- 
tunately not every examiner made this report, so the following 
study of the curve locations of the “reported similar’’ pairs is not 
as complete as it might be. Table 14 presents a summary of certain 
data that will be used in this part of the study. 

Table 14 has been so arranged that it shows the identification 
number of each of the pairs that was reported similar, and in ap- 
propriate columns the various scores and score differences. These 
data can be studied from the point of view of the degree of re- 
semblance shown by this group as compared with either the entire 
like-sex pair population or the entire twin population. It can also 
be studied from the standpoint of curve location for the various 
pairs under consideration. 

We shall note the matter of resemblance first. The following 
comparison furnishes rather striking evidence for the real similari- 
ty of those that are reported “similar.” Correlating by 


6=D? 
N (N’?’— 1) 
we have: 
For staniord-BinetrlL.OO. 2 ae. R = +.986 
BO Sache re ating ys sso cal eatesn ao R = +.940 
HUOT ATS GLANs, gMerrente Oe Wey abies se: Aateu «5 8% R = +.887 
SUMIMEN OP Li tol eens ers tate oe Pel aes R = +.987 


These are very high correlations. Not only are they high correla- 
tions, but with one exception they are materially higher than the 
results that were found in earlier parts of the study for the re- 
semblance in the entire like-sex group or the entire twin popula- 
tion. These figures are shown in Table 15s. 

This gives an excess of resemblance in favor of the “reported 
similar” in every case when they are compared with the total 
twin pairs, and in all cases except Beta when compared with total 
like-sex pairs. It is to be noted also that there is a steady increase 
in the value of r as the fraternal pairs or “supposed to be fra- 
ternal” pairs are dropped from consideration. 


44 CURTIS MERRIMAN 


TABLE 15 





TEACHER 





STANFORD 














BINET RATING 
a a ee Ee ir a Bes ara ee 
All twin pairs 10§ | :782'|voo. *}..g12 | 763) 841%] 149 ia Son 
All like-sex pairs 67 | .867 | 53 | 654 | 45 | .g08 92 | .925 
“Similar” like-sex pairs 92 | .986.] 22 »|. .940 | 18] 1887 12 | .987 



































For the discussion of curve location the reader must recall the 
facts presented in Table 9 and Figures B and C. It was there 
shown that the plotting of the score differences produced a highly 
skewed curve, the skew being towards a small score difference. If 
we define that portion of the Stanford-Binet curve which is pro- 
duced by score differences of 0 to 5 inclusive as the “Binet upper 
level,’ and that portion of the N.I.T. curve which is produced by 
score differences of 0 to 20 inclusive as the “N.I.T. upper level,” 
we can make the following observations: 

1. Of the 22 pairs reported similar, 13 pairs are on the “Binet 
upper level.” 

5. In the entire twin population, 9 pairs have a Binet I.Q. score 
difference of o and are therefore on the Binet upper level. Eight 
of these are like-sex pairs, and of the 8 pairs 5 are in the group 
of “reported similar.” No report as to similarity was received — 
upon the other 3 pairs. 

3. Of the 22 pairs reported similar, 12 pairs took the National. 
Eight of these are located on the “N.I.T. upper level.” — 


i, 


GENERAL SUMMARY OF PURPOSES, DATA, 
AND RESULTS 


Purposes. This study of the intellectual resemblance of twins 

has sought to answer three questions: 

a) What is the effect of environment upon the amount of in- 
tellectual resemblance of twins? 

b) Does the fact of twin origin and birth operate in any way 
to lower the intellectual level of a twin population? 

c) What light do the psychological data throw upon the cur- 
rent biological belief that there are two distinct types of 
twins, fraternal and duplicate? 

Data. Individual and group material was collected as follows: 


pal FOP -DINeu teSES) LOL re ae eet ye a cree IO5 pairs 
a eacier estimates TOPs. Fr. es ces et QO pairs 
PATIY VCtAtOStG TON a ctu niece sie’ 5 5 76 pairs 
National Intelligence tests for........ 143 pairs 


Treatment of Data. These data were studied from many dif- 

ferent angles. Young pairs were compared with old pairs. 

Like-sex pairs were compared with unlike-sex pairs. Boys were 

compared with girls, etc., etc. In making these various com- 

parisons four methods of treatment were used: 

a) Pearson correlations between various groups. 

b) Difference in gross scores. 

c) Curve plotting and fitting to determine character of popu- 
lation. 

d) Empirical study of correspondence between psychological 
data and judgment of friends as to the resemblance of cer- 
tain pairs. 

Findings. The results of the study are presented in the form 


_ of answers to the three questions asked at the outset. For con- 


venience, all the correlation results are assembled in Table 16. 
The reader will find it very helpful to refer frequently to this 
summary. 


46 


a) 


b) 


c) 


CURTIS MERRIMAN 


Environment appears to make no significant difference in 
the amount of twin resemblance. Table 16 shows twenty 
pairs of correlations on the basis of young twin pairs 
versus old twin pairs. Of these twenty pairs there are 15 
that show either very slight changes or changes that can 
be explained on the basis of small population. The larger 
changes of the teacher rating comparisons are explained 
on the basis of better acquaintance with the older pairs 
and over emphasis of slight differences. 

Twins suffer no intellectual handicap. This is shown in- 

various ways: wf 

1) Mean and median I.Q. practically same as for general 
population. 

2) Mental level of boys same as girls. 

3) Like-sex pairs same mental level as unlike-sex pairs. 

4) No significant differences in variability of sexes. 

5) Young pairs show slightly higher mental level but this 
is explained by the fact that Stanford-Binet is more 
difficult for older children. 

The data show quite conclusively that there are two dis- 

tinct types of twins. This is shown in various ways: 

1) In every case where like-sex pairs are compared with © 
unlike-sex pairs, the correlation of the like-sex pairs is 
significantly higher. Table 16 shows in groups 2 and 5 
the twenty-four correlations that provide the evidence 
for the statement just made. 

2) When sibling data are compared with twin data, the - 
correlations lie much nearer the unlike-sex pair twin 
data than to the like-sex pair data. This is in harmony 
with the biological claim that genetically speaking fra- 
ternal twins are siblings. 

3) All the curves and curve fitting tests used in the study 
indicate clearly a difference between like and unlike-sex 
pair twins. 

4) The empirical study of verbal reports on “similar 
pairs’ tends strongly to show that curve differences are 
to be largely accounted for by the like-sex pairs that 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


47 


show great intellectual and physical similarity, and that 
presumably belong to the “duplicate” type. 


All twin pairs 
Pairs, 5to gyrs. 
Pairs, 10 to 16 yrs. 





Like-sex pairs 
Like-sex 5 to g 
Like-sex Io to 16 








TABLE 16 


’ SUMMARY OF CoRRELATIONS 


BINET 
r 


-782+ .025 
.809 +.032 
275] #037 


867 + .020 
8824 .028 
865 +.027 


















































|N 


39 
gI 


53 
30 








Girl-girl pairs 
Girl-girl 5 to 
Girl-girl 10 to be 





Boy-boy pairs 
Boy-boy 5 to 9 
Boy-boy Io to 16 





Unlike-sex pairs 
Unlike-sex 5 to 9 
Unlike-sex 10 to 16 





| 857 £.029 
915 + .026 


.814+.050 


877 + .030 
.800 + .078 


890+ .034 


-504+.081 
774. + 064 
.298 + .137 





















































BETA | N.I.T. | TEACHER 
SON ODES a EE 
105| .841+.022 | 76] .891+.011 |143] .512+.053 
47] -784+.049 | 28] .797+.034 | 54] .686+.057 
58] .664+.054 | 48] .875+.017 | 89] .373+.081 
67| .go8+.017 | 45] .925+ .009 | 92] .654+.053 
2g] .921+.025 | 16] .946+.012 | 31] .788+.053 
38] 842 + .036 | 29] .865 + .022 61} .568 + .083 
| 40] .866 +.033 | 25] .928+.012 | 61] .645 4.071 | 
Ig] .709+.112 | 9g] .g65+.009 | 24] .g13+.030 
21] .896 +.032 | 16] .g1g+.021 | 37] .521+.123 
27] .938+.015 | 20] .g25+.018 | 31] .605 +.090 
O} -934+.049 | 7] -921+.041 | 7] .534+.161 
17] .747 +.080 | 13] .895+.027 | 24] .715+.089 
38] .732+.056 ele 025 266 + .102 
18] .§19+.147 753 +.066 z 681 +.090 
20] .643 + .OgI 834 .044 .O72 4.141 














16 


33 
9 


37 
16 


21 


-_ 
: 


iy 


as 


10, 


BIBLIOGRAPHY 


Bateson, WILLIAM. Determination of Sex. Nature, Feb. 3, 1921. 
CaTTELL, J. M. Statistical Study of American Men of Science. Science, 
New Series, Vol. XXIV, pp. 732-742. 

ConkKLIN, E.G. Heredity and Environment. 

Coss, Mary V. Evidence Bearing on the Origin of Twins from Single | 
Ovum. Science, April 12, 1915, pp. 501-2. 

DanrortH, C. H. Is Twinning Hereditary? Journal of Heredity, Vol. 
VII, (1916), p. 195. 

Davenport, C. B. The Influence of the Male in the Production of Human 
Twins. American Naturalist, March-April, 1920, pp. 97-122. 

Davenport, C. B. Inheritance of Temperament. 

DaAvENPORT, ET AL. Twins. Journal of Heredity, December, 1919. 
Doncaster, L. The Determination of Sex. A Review of Heredity, Vol. 
VI, June, 1915, p. 260. 

Dott, E. A. Psychological Measurement of Thirteen Pairs of Feeble 
Minded Siblings. Training School Bulletin, May, 1918, pp. 45-47. 
Gatton, F. Inquiries into Human Faculty. Everyman’s Library, pp. 155- 
172. 


. Gorpon, Kate. Report on Psychological Tests of Orphan Children. Jr. 


of Delinquency, Jan. I, 1919, pp. 46-56. 

GresELL, ARNOLD. Mental and Physical Correspondence in Twins. The 
Scientific Monthly, April and May, 1922, pp. 305-331 and 415-428. 

Haypven, C. C. A Case of Twinning in Dairy Cattle. Ohio Agricultural 
Experiment Station Bulletin, March-April, 1922, pp. 54-57. 


. Jorpon, H. E. A Note on Twinning, Jr. of Genetics, Vol. IV, 1914-15, 


pp. 79-81. 


. Newman, H. H. The Biology of Twins. University of Chicago Press, 


1Q17, pp. 1-185. 


. Ottver, JAMes. The Hereditary Tendency to Twinning. Eugenics Review, 


Vol. IV, (1912-13), pp. 39-53 and 154-167. 

Pearson, Kart. Concerning Inheritance, etc. Biometrika, Vol. III, pp. 
131-190; Vol. V, pp. 105-146. 

StarcH, Danret. Similarities of Brothers and Sisters in Mental Traits. 
Psychological Review, May 1917, pp. 235-8. 

THorRNDIKE, E. L. Measurement of Twins. Archives of Philosophy, Psy- 
chology, and Scientific Methods, Number One, September, 1905. 


. THompson, J. A. Heredity. 


Relative Number of Twins and Triplets. Science, March 18, 1921. 


. Fisuer, R. A. The Genesis of Twins. Genetics, Sept. 1910, pp. 489-490. 
. Wiccam, A. E. What Twins Tell Us About Ourselves. Physical Culture, 


October and November 10921. 


25. 


31. 
32. 


THE INTELLECTUAL RESEMBLANCE OF TWINS 49 


Two Kinds of Twins. Literary Digest, Vol. LII (May 27, 1916), pp. 
206-9. 

SmitH. Twins. Science, Vol. XXVII, p. 451. 

Partial Twin. Literary Digest, Vol. XLIV, p. 588. 

BALLANTYNE. Antenatal Pathology. 

GouLp AND Pyte. Anomalies and Curiosities of Medicine. 

Wiper, H. H. Duplicate Twins and Double Monsters. American Journal 
of Anatomy, Vol. III (1904). 

Wiper, H. H. Palm and Sole Studies. Biological Bulletins, Vol. XXX. 
Liu, Frank R. Problems of Fertilization (1919). University of Chi- 
cago Press. 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


n 
ms 
* 


SE’ SUS IETS IER EINER SIE SRI I es 


APPENDIX A 


Gross ‘Scores FoR VARIOUS ITEMS 


GRADE 


2b 
2b 
Ib 
Ib 
1a 
1a 
Ia 
Ia 
Ia 
Ia 
1b 
Ib 
Ib 
Ib 
Ib 
Ib 
1a 
1a 
2a 
2a 
1a 
2b 
1a 
Ia 
2a 
2a 
2b 
2b 
1a 
1a 
2a 
2b 
ra 
Ia 
tb 
Ia 
2b 
2b 


CAS 
5-10 
5-10 
6-11 
6-II 
6-3 
eo | 
6x 
6- 
6- 
6- 
6- 
6- 


PUM oOeFFNN HH 


M.A. 
6- 6 
6- 8 
7- 6 
74 
6- 4 
5- 4 
6-10 
7- 6 
6- 4 
6-10 
5- 6 
6- 0 
6- 2 
5-8 
7-1 
6-10 
6-10 
6- 6 
7- 4 
7- 6 
7-11 
8- 0 
5- 2 
4-10 
6-10 
6-10 
7- 2 
6-0 
7-8 
7-10 
7- 2 
7-0 
7- 8 
7- 8 
6- 3 
6-7 
Wi i 
8- 6 
7-10 
8- 8 


1.0. 


112 
114 
108 
106 
IOI 

85 
107 
117 

06 
104 

90 

08 
102 

04 
110 
106 
107 


TEACHER 


1.5 
2.2 
2.7 
3-3 
3.0 
3.0 
i 
2.6 
3.7 
2.7 


ae 
3.1 
2.5 
2.0 
2.5 
2.4 
3-4 
2.8 
At 
4.5 
3.1 
3.4 
3-4 
3-3 
2.8 
2.5 
2.9 
2.7 
2.6 
3.1 


BETA 


39 
45 
32 
30 


25 
18 


N.LT 


56 
88 


128 
108 


Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
\efiip 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pasar 


Pair 


21 


113 


22 


23 


24 


25 


26 


27 


28 


29 


30 


31 


32 


33 


34 


114 


115 


116 


117 


118 


119 


120 


121 


122 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


a) 
iz] 
ras 


AEDITE AAI AYIS SHIITES TST SPSS SII Ate Meda aaa yy eS 


GRADE 
Ib 
Ib 


Ca: 
7- 0 
7- 0 
7-10 


1.Q. 


86 
79 


TEACHER’ BETA 


3.8 
3.4 


2.3 
2.0 
Patil 
2.2 
3.4 
3.3 


2.8 
2.3 


2.9 
2.8 
3-5 
3-3 
3.0 
3.0 
3-5 
3-3 
21 
2.4 
21 
2.4 
3.8 
2.9 


2 
12 


53 


48 


47 
48 


51 


N.1.T. 


143 
134 


144 
132 


76 
150 


m 
ie] 
ra 


SA AAA ye Se eS Se a SS Sf 


ge Mo Fac a at 4 


GRADE 


3a 


CURTIS MERRIMAN 


CrAG 
8-11 
9- 6 


Oo 
' 
CHW ADRK WMDOOUUNUNOOO 


ooo vo © oO 
DOR Be ae aL 7 
NN 


Lon] 
Lon! 


o 
7 
or 


GSES OPP FP9 FS 
WwWWwWARKDOO A 


TEACHER 
2.9 
3.8 
2.6 
2.6 
3.2 
3.0 
4.5 
ke 
2.2 
ZY 
2.6 
3.2 
3.3 
3.4 


3.6 
a 
3.0 
3.1 


2.1 
2.1 
2.9 
3.3 
3.0 
3.0 


BETA 
53 
41 
61 


50 
41 


4I 
63 


N.LT. 
104 
214 
198 
268 
286 
114 
218 


240 
258 


174 
138 


108 


ete, 


Pair 
Pair 
Pair 
Pair 
Pair 
Barr 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 


Pair 


Pair - 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


132 


133 


134 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


n 
cl 
* 


HIGGS ASUS AAAS TIA de Ae TS INS S Ay de de dae yee eS SS yD Hy 


GRADE 


C.A. 
9-10 
9-10 
Q-II 
Q-II 
9Q- 2 
9Q- 2 
10- 6 
10- 6 
10-10 
10-10 
I0- 
10- 
10- 
10- 
10- 
10- 
10- 


i 
fo) 
7 
NOWW HH OAOH Be ew 
% * 


M.A. 


ai'oh. eer ¢ 
tse ee 
ee eee 
se eee 
4). 0h ee 
se eee 
eee ee 
se eee 
eens 
see ee 
eee ee 
see ee 
eeeee 
CRY oat TE 
see ee 
ee eee 
te eee 
is G86 


1.Q. 


TEACHER BETA 


222 
Be 
20 
3:5 
3.0 
2.8 
2.6 
3.4 
3.2 
2.2 


74 
72 
39 
54 
47 
70 


250 
232 
218 
262 


207 
178 
192 
169 
133 
156 
171 
148 
147 
139 
189 
197 

72 

74 
109 
189 
137 


54 CURTIS MERRIMAN 


SEX GRADE C.A M.A I.Q. TEACHER BETA N.LT. 
F 4a TO-80y fas a tex a 136 
Pair 62 M 4a II-I1 8- 6 71 4.1 44 116 
F 6b II-II 9-10 83 3.0 60 204 
Pair 63 M 6b II-10 II- 2 04 2.9 68 214 
F 6b: II-10 II- 5 06 39 50 206 
Pair 64 M 3b 11-8 9-10 84 3.8 ie 
M 3b 11- 8 7-11 68 3.8 
Pair 65 M 3a II- 5 9- 9 85 
M 3a II- 5 9- 0 79 
Pair 66 M 7b II- I I5- 5 139 
M 7b II- I I4-II 134 ts a 
Pair, 67. F II- 7 8- 6 73 59 140 
F “7 II- 7 8- 6 ve en 49 146 
Pair 68 F 7b II-II 12-5 104 2 74 204 
F 7b II-II 13- 6 113 3.9 80 . 324 
Pair 69 M Sa II-II 11- 8 90 3.0 76 196 
F 6b II-II 13- 0 III 7 So =. ../202 
Pair 70 M 6c II- I II- 2 IOI 3.0 38 158 
M 6c II- I Tise7 104 3.0 46 152 
Paineyt M 6a II- 5 I2- 4 108 2.4 73 258 
M 6a II- 5 12-II 113 2.8 66: 5/262 
Pair 145 F 6a 6 bee Pe . eke 196 
' F 6a TELS See ee 4 185 
Pair 146 M 7a Tiss?“ Vegineeey 223 
F by ph Caine Pad lM ma 255 
Pair 147 F sb Ti-cE er ees 135 
M 5b TI=g 12> Vt oe Me a. ye 139 
Pair 148 F 6a TEsgAC ha Petes Phage ut ee 274 
F 6a TERE ideas 204 
Pair 149 F 6b TT=: 7 oe 242 
M 4b TI) TA tee 195 
Pair 150 F 7 TI-2ciee eae 253 
F 7 Theos) eee ee 257 
Pair 151 F 6b Tints seal 202 
M 7b TLSSe) ur Ves ‘ 246 
Pair 152 F Sa Bio. SA wih ae 131 
F sb Tits Boh ae 183 
Pair 153 F 5a Df ir Pale Og yr Nte 169 
M 6b VUAlT io ies 269 
Pair 154 F 4a TI fiche I4I 
F 4a Tiss Bee ctl 1a tae 124 
Pair 92 F 6a 12- 7 II- 0 87 a2 65 oan 
F 6b 12-17) 10-10 86 RYE: 62 
Pait 173 M sb 12- I II- 2 92 3.0 62 
M 5b 12- I 10-11 OI 3.0 59 A: 
Paina M 6a I2- 0 II- 4 04 3.1 78 200 , 
F 6a 12- 0 12- 6 104 pa 82 286 





Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Pair 


Bar 


Pair 


Pair 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


SSAA eIddie gadis eee rye eee ee te ee Re eS 


= 


GRADE 
7b 
7b 
4a 


Cc, 


I2- 


tin tn “tn ta (SO °O (ONO i 6-6 Gaee: 1C at. it 


H 
to 
' 
_ 
° 


4 
to 
' 
4 
o) 


OOO STS] fe eH OS Gorm Go BR ow Gu OV OO 


see ee 


ee eee 


sees 


oad: ela) 


ee eee 


eens 


ee eee 


ee eee 


see ee 


ee eee 


eee ee 


see ee 


see ee 


sen ee 


1.Q. 


95 
96 
67 
Vie 
85 
85 
116 
82 
86 
82 
74 
79 
121 
134 
100 
75 
90 


TEACHER BETA 


3-4 
4.3 
3-5 
3-5 
2.6 


68 
7O 
54 


55 


N.1LT. 


126 
142 


170 
148 


94 
124 
254 
264 
264 
254 


213 
215 
209 
247 
I4I 
137 
266 
233 
117 
200 
178 
125 
259 
263 
169 
203 
241 
238 
263 
241 

22 


56 CURTIS MERRIMAN 


SEX GRADE C.A. M.A. I.Q. TEACHER BETA N.LT. | 
M 3b £2750) i Yeaos s tse wea ¥ 51 
Pair 166 M 5a 2-72" > ahs Aik oe is 223 
M Sa oe a ati aa me 235 
Pair 167 F 6a T2AZ" eat: ive ‘er we 221 
F 6a T2-"2ixi polity c 4 as Ag 201 
Pair 168 F a B2=°5 Ll Pade Wave oe < 248 
F 8b £2258 Ulweac's tats rt ai 277 
Pair 169 F 7b ae el ne ah 25 274 
F 7 52-935) Wty a a ae 308 
Pair 170 F 7a £B-18'7) ye “ahoeres ex Bae i 281 
F 7a aA iN ks ee suas rf os 305 
PA a7t F 5b 1 ae ae CAE oe eke een a I5I 
F Sb 12-4 Aes ahs, 4y os 159 
Pair 172 F 6a ioe FW Gee by 4 ba | eA 
F 6a Fon07 =) he, Nay ee Ae 235 
Pair 173 M 8 oy Me ae ae a ae 285 © 
F | TO AEA a heed sere: Sek nae as Rel OA! 
Pair 174 F 7a 12-19 TVs tes Lee “e 219 
F 7a T2505 ecu en tie ae vs 225 
Pair 88 F 8a 13- 4 I4- 7 109 2.5 84 
F 8a 13- 4 16- 3 122 2-57. 78 
Pair 89 M 8a 13- 9 13- 6 98 2.6 60 300 
F 8b 13- 9 I2- 0 87 3-4 73 300 
Pair 90 F 7a 13- 2 12- 6 905 3.5 51 256 
iB 7a 13- 2 9- 9 74 4.2 53 200 
Pair 91 F 7a 13- 4 II- 9 88 3.2 heh 224 
F 7 13- 4 13- 5 IOI 25 89 264 
Pair 92 M 8a 13- 8 13- 4 07 3.8 GG 200 
M 8a 13- 8 13- 9 100 2.9 62 226 
Pair 93 M 8b 13- 8 16- 0 117 2.2 82 206 
M 7a 13- 8 12- 8 93 2.6 85 - 280 
Pair 94 F 8a 13-10 I4- 5 104 80 206 
F 8a 13-10 I4- 3 103 78 2096 
Pair 95 M 7a 13- 6 13- 2 98 3.0 74 298 
M 7a 13- 6 M3 08 2.9 69 320 
Pair 06 M 8a 14- 4* 13-10 07 2 78 330 
F 7b 14- 3* 13- I 92 3.0 59 258 
Pait 67 M 5a Df ae Lal Ee 3.6 ey 
M 5a UR Dy ans ta 3.9 44 
Pair 98 F 5b 13- 2 10- 7 80 3.5 66 180 
F 5b 13- 2 10- 8 81 3.2 54 202 
Pair 176 °F 6a Rae aU Cees es 202 
F 6a T3552) eee 240 
Painet77 M 8a T3oh Oia) MCRAR ees 337 
F 7b TSO ow Anos tok 222 
Pair 178 M 8a 1K Ce” MPM 6 pk 266 
M 8b LO=040 8 ite ac 190» 





Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
sae 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 
Pair 


Pag 


THE INTELLECTUAL RESEMBLANCE OF TWINS 


n 
rel 
va 


MISSES SIS NS ee ee ee ee ye ee Oe eee nh ee 


GRADE 


8a 


C.A. 
13-II 
13-11 
I3- 5 
13- 
13- 
13- 


H a om mH 
w& Cones 
I woe 


_ 
oa 


=x = & 
ses 


14- 
14- 
14- 


14- 
14- 


NON ONO 00 CA tH OO ON ON OR 


= aere/e 
Pie elss6 
eee ee 
oe ae: 8 
sees 
eee ee 
eee ee 
Sari Ta 
ee eee 
eee ee 
een ee 
eee ee 
se eee 
tees 
ee eee 
ee ee 
eee ee 


13-10 


ee eee 
eee ee 
eee ee 
enews 6.8 
eee ee 
eee ee 
oe eee 


L.Q. 


TEACHER 


3-4 
3.4 
4.2 
4.0 
3.3 
4.0 
2.4 
4.4 
4.4 
4.3 
3-5 
3.4 
3.3 
2.8 
4.2 
3.5 
1.8 
1.5 


BETA 


of 


N.LT. 
273 
270 
304 
246 
269 
157 
295 
209 
219 
252 
264 
308 
216 
231 
232 
224 
304 
290 


58 


203 


204 


AAS NS de a Dh ee ey af af ty 


CURTIS MERRIMAN 


16-II 


eeeee 


eeeee 


eeeene 


eeeee 


eeeee 


eee ee 


oe eee 


eoeee 


se eee 


eeeee 


eee ee 


eeeee 


sens 


1.Q. 


TEACHER’ BETA 


3.7 
‘28 
3.5 
23 
2.8 
3-7 


3-5 
3-5 


70 
79 


N.LT. 
217 
188 
209 


104 


BA. 








4 Shih iN ais: 
ey 





* Hat MY f 
ig frie ee ‘ie 


Sane) 


BF21 .P96v 
oe effect “i ele basa ” maze 


il LA 


1 1012 00008 5433 





